The present invention relates to a communicating system, a communicating method, and a base station that transmit for example multimedia data from the base station to a vehicle and data from the vehicle to the base station.
In the inter-vehicle communicating system, there are needs for multimedia communication. In the multimedia communication, mobile stations send requests for information to the base station so that it downloads multimedia data such as video data and music data to the mobile stations. The multimedia communication has a feature of which the amount of data that are transmitted (uplinked) from a mobile station to the base station are smaller than the amount of data that are transmitted (downlinked) from the base station to the mobile station. Thus, to accomplish the multimedia communication with the inter-vehicle communicating system, a radio communicating system that can effectively transmit a large amount of data that are generated as burst data to mobile stations is required. In addition, when photographed video data are transmitted from a mobile station to the base station, it cannot be always said that the amount of photographed video data is small. Thus, it is desired to effectively transmit video data.
A communicating method that is suitable for an inter-vehicle multimedia radio communication. See, Japanese Patent Laid-Open Publication No. 2003-234688. In the communicating method thereof, data are transmitted and received with time division multiple access (TDMA) frames. A TDMA frame of the downlink channel is composed of a frame control message slot (FCMS) and a message data slot (MDS), whereas a TDMA frame of the uplink channel is composed of an activation slot (ACTS) and an MDS.
A communicating system that can transmit and receive a large amount of data in a relatively short time and that can simplify a communicating process and a circuit scale of the mobile station side has been provided using the communicating method of Patent Document 1. In this communicating method, since a predetermined amount of data is returned as a response signal, data that are normally transmitted are the same as data that are re-transmitted. Thus, the capacity of a buffer on the mobile station side can be decreased. In addition, processes performed on the mobile station side can be simplified. Data that can be sent at a time are for example an IP packet that is used in the Internet. Normally, the mobile station side and the base station side basically communicate on the basis of the TCP/IP.
However, in this communicating method, since the number of slots of a TDMA frame is fixed, if the status of the radio communication changes, data cannot be transmitted and received corresponding to the changed status. Thus, effective data transmission and data reception cannot be accomplished.
For example, one TDMA frame of the uplink channel has one ACTS (four sub slots). The ACTS is used for registration—deregistration request or connection setup—connection release request that the mobile station side sends to the base station. Even if the mobile station side does not send such a request at all, since this slot is allocated, the frequency band is substantially wasted.
In contrast, when the mobile station side frequently sends registration—deregistration request and connection setup—connection release request to the base station side, if the number of ACTSs is fixed to one, slots used for registering mobile stations and setting up connections become short. As a result, these processes will largely delay. Thus, the efficiency of the communicating system will remarkably deteriorate.